studies on the extraction behaviour of zirconium from mixed aqueous media with tbp

Journal o f Radioanalytical Chemistry, Vol. 29 (1976) 99-107

STUDIES ON THE EXTRACTION BEHAVIOUR OF ZIRCONIUM FROM MIXED AQUEOUS MEDIA WITH TBP

R. SHABANA, F. HAFEZ

Nuclear Chemistry Department, Atomic Energy Establishment, Cairo (Egypt)

(Received July 1, 1975)

A systematic study is presented on the extraction of Zr(IV) by tri-n-butylphosphate from H 2 SO~, HNOa, HC1 and HBr solutions as well from binary mixtures of H 2 SO4 and halogen acids. It has been found that Zr(IV) extraction is appreciably increased by addition of halogen acid to sulphuric acid solutions. On the other hand, the presence of water-miscible alcohols and acetone was found to enhance Zr(IV) extraction from HC1 and HNO 3 solutions. An investigation of extracted Zr complexes from pure acids and mixed aqueous media enabled an explanation of the observed enhancements. In the light of the obtained results, an extraction mechanism is suggested.

In t roduct ion

The isotope 9 5Zr is one o f the most important contaminant isotopes, as it has

relatively long half-life and high energy gamma-radiation. The separation of this

isotope from other elements and isotopes (par t icu la r ly fission products) is, therefore,

important in the processing and reprocessing of nuclear fuel, as well as in the treat-

ment of radioactive wastes. The reagent TBP is proved to b e an excellent solvent

for Zr ec t rac t ion) The extract)on of Zr from aqueous solutions containing two

different inorganic ligands with TBP was studied by many authors. 2'3 From HC1-

HNOa mixture, it was reported that Zr extraction is more efficient than from t h e

separate solutions of each acid. I t was also found that the presence of rather low

concentrations of hydrochloric or hydrobromic acids appreciably increase the ex-

traction o f some elements from sulphufic acid solutions. 4-6

Moreover, the addit ion of water-miscible alcohols and acetone to the aqueous

phase in a liquid-liquid extract ion system was found to greatly influence the ex-

traction o f metal salts from acidic solutions. 7-1 o

The present work summarises systematic investigations on the extract ion of 9 SZr

with solutions of TBP from HC1, HBr and H2 S04 as well as binary mixtures of

H2SO 4 and halogen acids. The effect of water-miscible alcohols and acetone on the

extract ion o f this isotope from HC1 and HN03 solutions was also studied. The two-

J. RadioanaL Chem. 29 0976) 99 7*

R. SHABANA, F. HAFEZ: STUDIES ON THE EXTRACTION

fold aim of this work is to open a wide scope for the selective extraction and stripping of this element as well as to explain the observed extraction enhancement.

The extraction mechanism is discussed in the light of the obtained results.

Experimental

Materials

Tri-n-butylphosphate (TBP) a product of B.D.H. was used after purification from mono- and di-esters by washing with 5% sodium carbonate solution and then with water. 11 Solutions of TBP with the non-polar diluent toluene (Reagent grade) were

prepared by volume. HNO3, HC1, HBr and H2SO4 were all a,alytical grade acids.

Isotope solution

The isotope 95Zr was prepared by the dissolution of irradiated analytical grade

reagent (ZrOC12) in few milliliters of 6M HC1 solution. Zirconium was then puri- fied from 9SNb by extraction into xylene solution of 0.5M TTA as reported. 12 Irradiation was carried out in the Egyptian Research Reactor (RRI) at Inshas for 48 hrs.

Apparatus

After necessary separations, the purity of 9SZr was checked by pulse height analysis with a Nuclear Chicago multichannel analyser ,(type 132B) connected to a Ge(Li) detector. An EKCO counting assembly connected to a well-type NaI(T1) crystal was employed for measuring the gamma activity of 9SZr.in solution.

Procedure

The extraction experiments were carried out in stoppered glass tubes by shaking equal volumes (3 ml) of both organic and aqueous phases of the appropriate acid and organic additive (alcohol or acetone) concentrations. Mixing with concentrated sulphuric acid was done while cooling in an ice bath. The tubes were shaken for 30 rain at room temperature (20 + 4 ~ Fluctuations of temperatures in such range have practically no effect on the extraction in our experiments. Also, studying the influence of shaking time on the extraction equilibrium has shown that ~5 min is sufficient for reaching equilibrium. After disengagement, aliquots of both phases were withdrawn for further analysis. The distribution ratio D was determined ex- perimentally as the ratio of the activities of the tracer in the organic and aqueous phases.

100 J. Radioanal. Chem. 29 (1976)


Results and discussion

Extraction of Zr(1V) from pure acid solutions

As preliminary studies in this work, the extraction of 95Zr with some non-polar

diluents (benzene, toluene, xylene, and hexone) were investigated. It was found

that the extraction of Zr(IV) from HCI, HBr, HNO3 and H2SO4 solutions of all

normalities gives values of D less than 0.001 for all the mentioned diluents except

Table 1 Extraction of Zr(IV) with 50% TBP in toluene from various acids

HC1, M D HBr, M

0.56

1.68

2.80

3.92

5.6

7.28

8.96

<0.01 0.32

<0.01 0.96

0.01 1.60

0.014 2.24

0.22 2.88

70.0 3.84

92.0 4.8

D

<0.01

<0.01

<0.01

<0.01

<0.01

0.018

0.033

HNO~, M

0.8

2.4

4.0

5.6

7.2

9.6

10.4

D

<0.01

<0.01

0.012

0.05

7.0

106

151

H 2 SO 4 , M

0.9 <0.01

2.7 <0.01

4.0 <0.01

6.3 0.03

8.1 0.10

9.0 0.28

10.8 0.70

hexone which has considerable extraction. Toluene was then chosen as the diluent

for TBP in our investigations.

The extraction of Zr(IV) with 50% TBP was carried out as a function of acid

concentration. The results obtained are given in Table 1. It is to be noted from

this table taht a quantitative extraction of Zr(1V) can be achieved from concen-

trated HC1 or HNO3 solutions and stripping can be effected from dilute solutions

of these acids. This characteristic behaviour can be used for its separation from ele-

ments behaving in the opposite sense. Extraction of 9SZr from H2 SO4 or HBr so-

lutions is low or negligible within the studied range of acidity. The extractable metal complexes seem to have the order nitrate > chloride > sulphate > bromide in

accordance with their stability.

Extraction of Zr(IV) from binary mixtures of acid solutions

The extraction of Zr with 50% TBP in toluene from aqueous solutions containing

two different inorganic ligands was studied. It has been found that the extraction

from mixtures of HC1 or HBr and H2SO4 solutions is much higher than from sep-

arate solutions of each acid. The obtained results are represented on Figs 1 and 2.

J. Radioanal. Chem. 29 (1976) 101


DI 100- -

3 2

1 10

I0-'

i0 -2

Fig. 1. Effect of HCI concentration on the extraction of Zr(IV) with 50% TBP from: 1 - 4.5M H2SO4, 2 - 7M H2SO 4 and 3 - 9M H:SO,

Extraction of Zr from sulphurie acid is seen to increase considerably with increasing

hydrochloric or hydrobromic acid concentrations. In order to find out the opt imum

conditions for Zr extraction, the effect of HC1 or HBr concentration on D was car-

ried out at 9, 7 and 4.5M sulphuric acid. It was found that a value of about 13

was obtained for D at a mixture of 4.5M H2SO4 - 5M HC1. Therefore, practical

distribution ratios can be reached without the necessity of using high sulphuric or

hydrochloric acid concentrations. Subsequently, Zr may be quanti tat ively extracted

from 9M H2SO4 solution in presence of 5M HC1 or 3M HBr.

The observed enhancement in Dzr when working in H2SO4 - h a l o g e n acid mix-

tures is in accordance with our previous work on the extract ion of some actinides

from similar solutions. 6'!3 The increase in distribution coefficients of these elements

was attr ibuted among other reasons to the formation of more soluble complexes

and the dehydrat ing action of sulphuric acid.



Ol

10

10 -1

3

2

HBr~ M

/ IO-: -

!

Fig. 2. Effect of HBr concentration on the extraction of Zr(IV) with 50% TBP from: 1 - 4.5M H2SO4, 2 - 7M H~SO 4 and 3 - 9M H2SO 4

In light of the above data one may suggest that halides of Zr form extractable

complexes with TBP and that these complexes are extracted from sulphuric acid

solutions possibly according to the following mechanism:

ZrX4 + nTBP ~ ZrX4 �9 nTBP (1)

It has been reported 14 that in the presence of excess acids Zr exists as anionic

complexes.

ZrX4 + n HX ~ n H § + ZrX~4-+n ) (2)

I f one of the two acids is part ial ly replaced by sulphuric acid, the lat ter will

help in removing water needed for p ro ton solvation, and this causes React ion (2)

to be shifted to the left hand side and, hence, higher extraction is obtained.



DI

100

p Ace(one

2_ pcot~ano I

�9 Et han0i t : M e t h a n o l

0 tO 20 30 i

~0 50 6o Additive ) %

Fig. 3. Effect of additive concentration on the extraction of Zr(IV) from: 5.6M HCI with

50% TBP

Extraction o f Zr(IV) from acidic - alcoholic or acetonic solutions

The addition of water-miscible alcohols and acetone tO the aqueous phase was

found to greatly influence the extraction of metal salts from acidic solutions. It

was therefore interesting to investigate the effects of these organic additives in the

present system. The effect of increasing amounts of these additives on the extrac-

tion of Zr(IV) with 50% TBP in toluene from 5.6M HC1 is represented in Fig. 3.

This concentration of the acid has been chosen in order to minimize errors in the

determination of D, if higher acid concentrations are used, it will lead to very low

counts in aqueous phase. It is clear from this figure that D is markedly increased by increasing additive concentration. The highest effect is due to acetone. In the

presence of 50% acetone, D increases to more than 1000 times of that in the ab-

sence of acetone. It can be said that quantitative extraction of Zr(IV) is possible


IL SHABANA, F. HAFEZ: STUDIES ON THE EXTRACTION

from intermediate HC1 concentrations by addition of organic additives whereas

stripping can be achieved from low acidity.

The effect of water-miscible additives was found to increase in the order: me-

thanol < ethanol < isopropanol < acetone. This behaviour is attributed to the de-

O,

I0 "1

~ Acetone Ethonol M~thonol

,0 ' ~ 1 7 6 ,. , .

Fig. 4. Effect of additive concentration on the extraction of Zr(IV) from 5.6M HNO 3 with 50% TBP

crease in hydration of zirconium ions with the increase of organic additive concen-

tration, which in turn provides a better chance for interaction with the chloride

ions to form the extractable chloride complexes. From a physicochemical point of

view the extraction of the element species into TBP solutions is governed by the

difference of the dielectric constant between the two phases which decreases by

the addition of alcohols and acetone to the aqueous phase. The sequence of extrac-

tion is in accordance with the dielectric constants of the different organic additives.

Fig. 4 represents the effect of the organic additives on the extraction of Zr(IV)

from 5.6M HNO3 solutions. An increase in the extraction coefficient due to the

increase of the additive concentration has been found. The effect of additives in

the nitric acid system is less pronounced than that in the HC1 system.



Dt 100 -

1 < 3 -

,10 -~

3

2

TBP,~ "1,,

Fig. 5. Effect of TBP concentration on the extraction of Zr(IV) from: 1 - 5.6M HC1, 2 - 5.6M HC1 + 4.5M H2SO4, 3 - 5.6M HC1 + 50% ethanol

Effect o f solvent concentration

The effect of TBP concentration on D values for Zr(IV) was studied in order to

determine the number of TBP molecules in the extracted Zr complexes. Log-log

plots of D vs. TBP concentrat ion at a constant aqueous phase composit ion are pre-

sented in Fig. 5. The values of the slopes for the obtained straight lines in this

figure imply that mixtures o f complexes are extracted containing 1 and 2 TBP mol-

ecules. However, it was reported 1,1s that the solvation number of the extracted Zr

complexes from HC1 and HNO3 solutions varies between 1 and 2. The similarity

of the slopes in case of HC1 or HC1 mixture with sulphuric acid or ethanol might

mean that the presence of sulphuric acid or alcohols in the aqueous phase does not

impose any considerable influence on the characteristics o f the extracted Zr com-

plexes.

106 J. RadioanaL Chem. 29 (1976)


References

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143 (.1962) 1413. 3. G. H. YAGODIN, G. E. KAPLAN, O. A. MOSTOVAYA, S. D. MOISEEV, L. P. DMITRIEVA,

Zh. Neorg. Khim., 8 (1963) 1973. 4. A. ALIAN, W. SANAD, Radiochim. Acta, 17 (1972) 158. 5. A. ALLAN, A. HAGGAG, W. SANAD, J. Radioanal. Chem., 20 (1974) 559. 6. N. SOUKA, R. SHABANA, F. HAFEZ, Radiochim. Acta, (in the press). 7. A. ALIAN, W. SANAD, R. SHABANA, Talanta, 15 (1968) 639. 8. A. ALIAN, W. SANAD, R. SHABANA, A. HAGGAG, Microchem. J., 16 (1971) 515.

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studies on the extraction behaviour of zirconium from mixed aqueous media with tbp

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